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After a fire, a criminal investigation is officially launched if arson is suspected or when suspicious human remains are found. The biggest challenge in such an investigation is the scene itself, and the fact that much of the evidence is likely destroyed. But, sometimes surprisingly, robust evidence can still be recovered.

Fingerprints: Fingerprints are an organic slurry of amino acids and fats mixed with inorganic compounds. As with most organic compounds, they are at high risk of damage or destruction from both radiant and direct heat, water from hoses, and the soot and ash accompanying the smoke plume and the actual fire. Patent impressions (prints visible to the naked eye, often a transfer from a substance like ink or blood) can sometimes survive, even when the surface on which they are found is scorched or charred but still intact. In fact, some prints become even more ‘set’ with the fire’s heat. Modern detection methods—like chemical developers and lasers—are so superior to previous forensic tools that they can expose even heat-denatured prints. In some cases, the heat will bake the print, causing the oils in it to darken, turning it from a latent print (invisible to the naked eye without some form of detection method) to a patent print.

Tool impressions: Materials melt at different temperatures during a fire. So, while wood will ignite in the presence of flame at 350oC, aluminum won’t melt until 660oC, steel at 1430oC, and iron at 1535oC. Tool impressions made in higher melting temperature metals may very well survive the fire completely intact.

Fragment matches: In non-fire scenarios, the physical matching of fragments—rope, tape, fabric, concrete or glass—can connect a killer to his victim, or can assist in establishing a sequence of events. Even fire-damaged materials, if the edges are not badly disintegrated, can assist with this aspect of the investigation. Most glass in a fire is shattered by force or thermal shock, but as long as the majority of the fragments can be recovered, reconstruction of the original structure and matching of adjacent fragments can still be achieved.

Trace Evidence: This type of evidence, depending on the type, is the most frequently lost as a result of fire. Nevertheless much trace evidence can still exist, based on protected areas on a body or in the scene. Fiber, hair, paint samples, and soil can all be recovered from fire scenes, given the right circumstances.

Blood/DNA: Unless the body is nearly or completely charred to ash, the opportunity exists to extract blood from one of the cardiac chambers as they are well protected by the torso during the fire. An alternative strategy is to harvest tissue from deep within the quadriceps muscle of the thigh for DNA extraction. In A FLAME IN THE WIND OF DEATH, Massachusetts Medical Examiner Dr. Edward Rowe suggests both of these methods while the team is trying to identify burned remains.

Next week, we’ll look more deeply into the damage fire does to the human body. When a victim is found in a fire, how can investigators identify the remains and determine the cause of death? We’ll be back next week with more…